Some legal writers advocate writing out a number and then adding digits in parentheses. In this post, Ken Adams argues against this practice. If anything, it makes what your writing more verbose and harder to read regardless of the type of document.

Most people do not argue with Strunk and White. Its 3rd edition says to spell out numbers under 100, and use digits for numbers 100 and above. The 4th edition, which came out in 2000, specifically admonishes against spelling out numbers, unless they are used in dialogue. -CCE

More often than not, contract drafters use words and digits to express numbers, as in no later than thirty (30) days after the Closing. That’s a bad idea, for two reasons:

First, it creates clutter that distracts the reader. And the more numbers a contract contains, the greater the distraction.

And second, it violates a cardinal rule of drafting—Thou shalt not state the same thing twice in a contract! Whenever you say the same thing twice, you introduce a potential source of inconsistency. . . .